The human skeleton is formed of bones, each bone performing a structural role, either individually or collectively with other bones. For example, the spine, which surrounds and protects the spinal cord and associated nerves, provides structure to the body, and enables fluid movement in many planes. Constructed of essentially twenty-four stacked vertebrae, the spine includes seven cervical vertebrae, twelve thoracic vertebrae and five lumbar vertebrae. A healthy spine is flexible in multiple directions to enable a broad range of physical movement. Intervertebral disks are disposed between adjacent vertebrae and provide cushioning and dampening to protect the spinal column and nerves in response to the various translational and rotational forces associated with body motion. Maintenance of the structural integrity and approximate axial alignment of the vertebrae is one key to good health.
A clinical subject's spine may be damaged or otherwise compromised in one of many ways. Abnormalities of or damage to the spine include but are not limited to scoliosis (abnormal lateral curvature), kyphosis, excessive lordosis, spondylolisthesis, displaced, degenerative or ruptured discs, fractures to one or more vertebral bodies and tumors. These and other possible spinal conditions directly and adversely affect mobility, and also cause moderate to extreme or even debilitating pain, at times accompanied by diminished or lost nerve function.
To ameliorate pain and restore loss of function associated with spinal conditions, a variety of conventional procedures have been developed using an array of mechanical surgical systems and implants that can secure two or more vertebrae in a relatively fixed position and can stabilize and straighten spinal deviations along the spinal axis. A stabilization system can be used without fusion treatment of the spine, or in conjunction with fusion treatment of the spine wherein one or more spacing devices is used to replace all or a portion of a vertebral disc. Typically, such discal implants are used together with natural bone components obtained from the clinical subject or a donor source, artificial bone, other biologic components to promote bone growth and fusion between the adjacent vertebral bodies. One or more such replacements may be accomplished in a spinal fixation surgery. The fixation system, with or without fusion components, operates to create a substantially rigid construct of bone and mechanical hardware that replaces damaged or diseased vertebrae and connects them to relatively healthier adjacent vertebrae.
Generally, spinal fixation systems involve some mode of stabilization using one or more rigid or substantially rigid surgical stabilization elements, such as a rod or a plate, and means for fastening and securing the stabilization element to bone. Fastening means can include one or more bone anchors, such as screws or bolts, assembled with connectors that enable engagement with one or more stabilization elements. The connectors may include hooks, clamps, cross connectors and other structures that engage with one or more of stabilization elements and anchors. These systems of anchor and connector assemblies and stabilization elements are secured to two or more vertebrae and are interconnected to provide support, encourage alignment or realignment of the vertebrae, and to achieve immobilization and fusion.
When spinal fixation surgery is performed from the anterior aspect of the clinical subject, it is conventional practice to affix a stabilization element in the form of a thin plate, typically formed of metal, to adjacent vertebral bodies and secure the plate using anchors, such as screws. When the fixation surgery is performed from the posterior aspect of the clinical subject, it is conventional practice to affix bone anchors into the vertebral bodies, typically in the pedicle. Multiple levels of adjacent vertebrae may be fixed in this manner. Interconnection of the secured anchors to the stabilization element creates a rigid fixation between the adjacent vertebral bodies.
The mode of surgical access may be open, that is, involving a relatively extensive resection of the soft tissue to plainly expose the vertebrae to be fixated. In some examples, the mode of surgical access may be minimal, wherein less invasive surgical techniques are used to minimize tissue resection. These less invasive approaches have many benefits to the clinical subject, however, the associated reduction in direct access and visualization of the vertebral tissue practically means that the anchor implants are difficult to access, grasp and manipulate with instruments, thus complicating the surgeon's efforts and often prolonging the amount of time that the clinical subject is in surgery.
Among the many challenges associated with placement of vertebral stabilization systems is the fact that adjacent vertebrae are typically not perfectly aligned. Indeed, along any particular portion of a spine, a series of adjacent vertebrae can deviate laterally a great deal from the central axis of the spine. Further, as a result of natural spinal curvature and any vertebral defects, corresponding portions, such as pedicles, of adjacent vertebra are not in the same plane. In the context of implanting spinal fixation systems, these variations can be accommodated to some extent by introducing bends or curves in the substantially rigid stabilization element(s) used for fixation. But in instances where the therapeutic benefit is obtained by realigning adjacent vertebrae, adjustment of the curvature of the stabilization element(s) is not a completely satisfactory solution. Accordingly, it is typically the case that the surgeon and surgical team must manipulate the spine and the system instruments in an attempt to align the secured anchors for attachment to a stabilization element. Often, the extent of nonalignment, both in terms of longitudinal and vertical planar positions of vertebrae along the spine, can cause failure of one or more of the system components, extend surgery, cause damage to the clinical subject's spine, and ultimately lead to a less than desirable clinical outcome. The challenges of access in minimally invasive procedures can compound the difficulties associated with non-aligned vertebral bodies.
Attempts have been made in the design of spinal fixation systems to address variability of spinal anatomy, such as those variations described above. In many examples of conventional systems, anchors are adapted to achieve a range of variability in positioning based on pivotal rotation of the anchor such that the axis of the secured anchor relative to the stabilization element can be varied. These are referred to as poly-axial and uni-axial anchors. They are useful in particular for facilitating attachment of a stabilization element to two or more vertebrae that are not aligned along the spinal axis. There are other examples of systems that are adapted with features that facilitate engagement of non-axially aligned vertebrae. But there are no conventional systems suitable for accommodating the variability in the relative height of adjacent vertebrae, wherein corresponding portions of adjacent vertebrae are not on the same plane. Further, there are no conventional systems that allow the surgeon the option to install bone anchors into the bone and then select from a suite of modular anchor components to achieve an optimized system for fixation that avoids or minimizes the problems associated with anatomical variations in the spine. Beyond the spine, such as for other bones and bone fragments in the body, there are likewise no systems that provide either or both modularity and length adjustability options in the fixation or reduction of bones and bone fragments.
To address the above-described challenges, there is need for bone anchors and other implants that meet or exceed the functionality of conventional anchors while also providing adjustability, and ideally, modularity, to address the height variability of vertebral bodies that do not share a common plane. Thus, what is needed, for example in the context of the spine, is a fixation system that includes one or more anchors that are capable of mono-, uni-, and poly-axial positioning and allow substantial vertical travel between the distal attachment point in the bone and the proximal position of a stabilization element, and are capable of locking to avoid further vertical travel after the system implantation is completed. Such an anchor would enable simplified attachment of adjacent anchors to a stabilization element by reducing the extent of height variability of adjacent anchors, thereby avoiding many of the challenges faced in the surgical setting.